Regardless of breed, when you are drawing a horse using guidelines you can count on the body being about three circles long, the front knees being lower to the ground than the back knees, and the general rule that a horse is about two circles tall, plus ankles and hooves. For the head, it helps to segment it into three sections; neck, cranium, and snout.
The three horses in the example shown were made with the same skeleton with minor tweaks depending on the breed. For example, the Arabian has a longer neck than the other two breeds, while the Shire has an overall larger frame.
To create believable horses, it's important to understand the differences between the breeds and then employ them in your drawing.